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# Which is the best battery for Quadcrawler

Posts: 262
edited 2005-02-10 - 23:28:49 in Robotics
As the power supply for the Quadcrawler I have been using a 6 cell pack Ni-Mh.·I use·six, 1,2V·2100ma cells.
That works pretty well.·

However after reading·battery issues in this forum I am wondering if·there is a more apropriate·battery pack.
I have tested the maximum load using an amp meter and a programmable power supply.
I see·there are times when the load exceeds·the maximum battery pack power.

By the way, is 7,2V too much for servos ?

·

• Posts: 3,815
edited 2005-02-09 - 19:40:43
1. The 2100 mA rating is actually a 2100 mA-Hour rating. This means these batteries should be able to put out 2.1 Amps for an hour -- or 1.05 Amps for 2 hours, or 4.2 Amps for 30 minutes. Actually, higher currents will get you less time, but that's the principle. I've heard of people completely discharging NiMH in minutes, and getting 10 or 20 Amps out of them. They get warm when you do that.

If you're pulling more than 2.1 Amps out of them for short periods of time, this should not do any damage.

2. 7.2 volts is high for a servo -- it will probably work, but will possibly shorten its life. Most servo's are designed for 4 x 1.5 volt Alkaline cells, or 5 x 1.2 volt NiMh (== 6 volts in both cases). A servo consists of both an electric motor AND some control electronics. Higher voltage will lessen the life of the motor, but can kill the control electronics. A couple of high-current diodes in series would drop the voltage, if you are concerned.· The best thing to do is go to the vendors web-site, and see what they recommend.

3. "Power" is measured in Watts. "Current" is measured in Amps. The "Power" equation is P = IV, or Watts = Current * Voltage. A "Load" is usually measured as resistance. Sometimes it is measured as "Power Used".

Having said that, I think it is impossible for the 'Load' to exceed the 'maximum battery pack power'. First of all, the NiMH battery pack will drive very small resistances -- it can put out tens of amps. It will need a recharge pretty quickly, but it will do it.

Frankly, it sounds like a 6-cell NiMH battery pack is a little 'hot' (voltage too high), but otherwise would be perfect for what you are doing.

What currents did you measure, at what voltages?
• Posts: 247
edited 2005-02-10 - 18:17:50
The key question is, as allanlane5 asked, "what currents did you measure...?" Anything under 150mA, and alkaline AA's will probably work fine. Anything more, and alkalines start having their capacity cut. For instance, as allanlane5 mentioned, a 2800mAh alkaline can put out 100mA for 28 hours, or 50mA for 56 hours, but at high drains, they "loose" capacity due to high internal resistance. Check out the graph at the bottom of page 2:

http://data.energizer.com/PDFs/e91.pdf

It's a log graph, which might throw you off, but the important thing is that it's not linear. AA's will work up to about 1A, but they won't last long AT ALL.

For higher current drains, use NiMH or NiCD. NiMH can be drained probably up to 2C (2 times the capacity of the cell/hr), or about 3.2A for a 1600mAh AA cell. High drains will compromise the life of the cell though. NiCD's are monsters for high drains (that's why they're used in about every power tool), but don't have as much capacity. Li-ion's are high capacity (for a rechargable), but suck for high current drains (ie, they're great for cell phones, but not power tools). AA Lithium alkalines (aka photo-oriented batteries) work well up to about an amp, have a high capacity, but are expensive (about \$10 for 4 at Walmart).

Hope that helps,
Dave
• Posts: 747
edited 2005-02-10 - 23:28:49
Don't go for the E2 Titanium batteries-the're the same as heavy-duty cells for a higher price
(ouch!)
bugg

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Really weird kid